The present invention relates in general terms to the procedures used in storing radioactive materials obtained from spent fuel elements following their discharge from a nuclear reactor.
Fissile materials which have been used in a nuclear reactor, such as e.g. a pressurized light water-cooled, uranium-enriched reactor, are depleted in U.sub.235 and correlatively enriched in plutonium and at the same time waste is produced. As the latter is fissile and can in turn be used in fast neutron reactors, reprocessing operations are frequently carried out on such spent fuel elements and essentially permit the separation of uranium depleted in isotope 235 and the plutonium formed, as well as the conditioning of the waste in a safe form. Following a reprocessing operation, the residual unusable products and which contain a large proportion of highly radioactive materials then undergo vitrification.
Two major problems have to be taken into consideration in connection with this storage process. Firstly and obviously, the thus conditioned waste is highly radioactive and constitutes a fatal hazard for all living organisms, from which it must be separated by biological protection means. Secondly, and this is often not taken into consideration with all the attention which should be taken, the radioactive disintegration reactions taking place therein, release energy in the form of heat. It must also be borne in mind that the decay periods of these radioactive materials are often very long and can extend e.g. to between 30 and 30,000 years.
To illustrate what has been stated hereinbefore, the following tables 1 and 2 give respectively for fission products and actinides, the masses and powers of the radioactive nuclei obtained on the basis of the reprocessing of one tonne of uranium contained in the fuel elements of a light water nuclear reactor, whose reprocessing took place 3 years after discharging the fuel.